Manufacturing methods for color filters used in color liquid crystal display devices include the dyeing method, the electrodeposition method, the printing method, and the dispersion method.
The dyeing method dyes a synthetic high polymer or a natural protein such as gelatin, glue, or casein to obtain a color filter. The color filter made by this method, however, however, has difficulties regarding storage stability in a moist environment, light resistance, and heat resistance, and in larger display devices, difficulty in uniform control of dyeing and adhesive characteristics leading to difficulties in the control of film thickness and unevenness of color.
In the electrodeposition method, color pixels are formed by applying a voltage to a transparent electrode, formed in advance in a specified pattern, and immersing the electrode in a bath having an ionized high polymer, which contains a pigment dissolved or dispersed in a solvent. The electrodeposition method requires a photolithography process, including transparent electrode coating for color filter forming, and an etching process, in addition to the forming process for the transparent display electrode. Any short-circuit results in a line defect, causing a drop in yield. Thus, electrodeposition is difficult to apply except in striped arrangements (which means that color filters are arranged in parallel), and it is difficult to control the transparent electrode film quality.
Printing includes such methods as dry offset printing and intaglio offset printing. But its use is limited because filtering of the ink is difficult due to its high viscosity, because defects are apt to occur due to dust, foreign matter, and the presence of the ink binder gel, and because there are problems of insufficient position accuracy and line width accuracy, and problems in surface smoothness. Thus, it is difficult to use for high resolution color displays having large screens.
The dispersion method forms color filters with pigment dispersed in a negative type photosensitive resin. The dispersion method can form colored pixels by a coating, exposure, and development process. The filter obtained by this method is stable against stresses such as light and heat because pigments are used. Further, position accuracy is adequate because patterning is done through use of photolithography. Thus it is optimal for high resolution color displays with large screen.
Japanese Published Unexamined Patent Application (PUPA) 63-128302 discloses a method in which colored pixels are formed by printing. Then pinhole defects are repaired. A black matrix is formed by exposing a black negative type photosensitive resin applied to the colored pixels from the transparent substrate side. The formed colored pixels are used as the photo mask.
According to the method of PUPA 63-128302, the black matrix portion and pinhole defective portions of the colored pixels will be of the same optical density, since the black matrix and the pinhole defects of the colored pixels are composed of and filled with black resist of the same material with the same film thickness. Because the light shielding by the black matrix requires an optical density of at least 2.0 (log 1/T, where T is the transmission), the pinhole defects may be perceived as black defects, especially when a static picture is displayed.
The paper by T. Fukuchi, M. Suginoya, H. Kawamori, and K. Iwasa titled "Self-Alignment Fabrication of ITO Electrode Pattern on Electrodeposited Tricolor Filter in Black Matrix: An Application to STN-LCD" published on pages 388 to 391 of JAPAN DISPLAY '89, on Oct. 16, 1989, discloses the formation of a black matrix between colored pixels formed by electrodeposition, by exposing, from the transparent substrate side, a black negative type photosensitive resin applied to the entire surface after forming colored pixels by electrodeposition, using the colored pixels as the photo mask.
The paper by Fukuchi et al. (JAPAN DISPLAY, 1989) proposes only a method of providing a black matrix in self alignment but never suggests repairing color filter defects. If pinhole defects caused by the photolithographic process of forming the ITO electrode or caused by the electrodeposit filter forming process are present, the pinhole defects will be filled with a black resist having the same optical density as the black matrix and can be perceived as a black defect, particularly when a static picture is displayed, as in the case for PUPA 63-128302. Therefore, the method of repairing the defective portion and forming black matrix using the same material at the same time merely results in changing white defects into black defects, which are, after all, also perceived as defects when displayed. Thus, this cannot be said to be a true repair.